In industry, polymeric resin films have a wide variety of applications including bases for packing, photographic films, condensers, electric insulators, labels, magnetic recording media, medical purposes. In recent years, polymeric resin films have also been in widespread use as base materials for picture display devices such as rear projection type screens, plasma display panels or liquid crystal display devices.
In particular, since a picture display device, unlike a cathode ray tube, adopts an indirect illumination system in which light irradiated from a fluorescent tube of a backlight provided on the backside of the device is reflected to a metal deposition layer or semi-transparent or opaque, white reflecting plate to display a picture through a light guide plate, visibility is poor, which becomes worsened according to pursuit of larger display devices.
To overcome the above-described problem, a light diffusing film that is adhered to a light guide plate to increase luminance by uniformly diffusing light, has been developed.
For example, Japanese laid-open patent Nos. hei 6-67003, 7-174909 and 9-127314, U.S. Pat. No. 5,706,134 and Japanese Utility Model No. 2,539,491 disclose light diffusing films having a transparent binder resin solution in which inorganic or organic particles are dispersed, coated on one surface of a transparent polymeric resin film as a light diffuser.
Here, either an inorganic light diffuser or an organic light diffuser may be used as the light diffuser of the present invention. Examples of the inorganic light diffuser include silica, zirconia, calcium carbonate, barium sulfate, glass, titanium dioxide, and so on, and examples of the organic light diffuser include fine particles of urethane based resins, acryl based resins, vinyl chloride based resins, and so on.
The diameter of the light diffuser is typically in the range of 1 to 50 μm. In order to maximize the formed of a poly of the light diffuser per unit area, particles having different particle sizes are generally used.
Also, in order to increase diffusion transmittance, there has been an attempt to maximize the ratio of a light diffuser to a transparent binder resin. However, in such a light diffusing layer, light diffusing materials cannot exist independently but flocculate with one another or exist in the form of laminates.
FIG. 3 is a cross-sectional view of a conventional light diffusing film. The transmittance of light transmitted through a light guide plate P is lowered at a portion L where light diffuser particles S are laminated. In particular, the transmittance is noticeably reduced at a portion where the particles are flocculated. Thus, there is a limit in improving the uniformity in luminance. Also, black spots are easily visible by naked eye, which makes the conventional light diffusing film unsuitable as a light diffusing film.